The present invention is directed to an electric lamp, and more particularly to a discharge lamp that is free of mercury and that contains a zinc iodide dopant.
Government agencies and the automotive industry have acknowledged concerns with automotive mercury use since the early 1990's. In 1995 it was determined that mercury switches were responsible for more than 99% of the mercury in automobiles—primarily in hood and trunk lighting, but also in antilock braking systems, Toxics in Vehicles: Mercury, A report by the Ecology Center, Great Lakes United, University of Tennessee Center for Clean Products and Clean Technologies, January 2001. As a result, the automakers agreed to voluntarily phase out mercury switches within a few years and to educate auto recyclers how to remove switches from existing vehicles. While the use of mercury in convenience lighting switches has significantly declined since 1996, mercury use for ABS applications appears to have at least doubled and possibly tripled. Other uses of mercury in automobiles, such as high intensity discharge headlamps, navigational displays and family entertainment systems, also appear to be on the rise.
High Intensity Discharge (HID) headlamps are an emerging application for mercury in automobiles. These headlamps offer improved visibility, longer life, and use less energy than standard tungsten halogen headlamps. Each HID light source contains approximately 0.5 mg of mercury and passes the Federal TCLP test for hazardous waste. The European Union (EU) ELV (end-of life vehicles) directive exempts mercury-containing bulbs from its ban on mercury in vehicles. The use of HID headlamps is expected to increase as introduction on less expensive, higher volume models continues.
It is reasonable to ask why mercury is present in an automotive HID lamp. Mercury does not significantly contribute to the visible spectrum during steady state operation since its lowest excitation levels are higher in energy than the ionization potential of the metal halide additives added to produce white light. Mercury is not essential to the operation of the halogen cycle except as a sequestering agent for excess iodine, which is always formed by chemical reaction within the lamp. The mercuric iodide resulting in the lamp is largely transparent to visible light. There are, however, several additional functions of mercury that make it extremely useful.
Mercury vapor determines the electrical resistance of the arc and is a thermal insulator around the constricted arc channel. The efficient operation of HID lamps with relatively high-pressure metal vapor requires a high total pressure filling to prevent rapid diffusion of dissociated metal and iodine atoms from the arc core to the tube wall. If dissociation took place primarily in the arc core and recombination took place primarily at the wall, the loss of energy due to the dissociation process would be very high, resulting in an inefficient lamp. Mercury is a convenient way of achieving a high total pressure for operation while still having a low pressure at ignition, so that reasonable starting voltages can be obtained.
If any free iodine vapor is present in the lamp at ignition, starting voltages are very high because the strong electron-attaching properties of iodine (I2) interfere with the Townsend avalanche formation, and the vapor pressure of iodine (I2) is relatively high at ambient conditions (0.4 Torr), W. P. Lapatovich and A. B. Budinger, Winkout in HID Discharges, Paper O1I4, IEEE Conference Record-Abstracts, 28th Conference on Plasma Science, PPPS-2001, Jun. 17-22, 2000, Las Vegas, Nev. The presence of mercury in excess then ensures that only mercury iodide (HgI2) is present at starting. Although mercury iodide (HgI2) is also an electron-attaching gas, its vapor pressure is substantially lower (<10−3 Torr) and causes only a moderate increase in starting voltage.
The advantages of mercury—a large potential gradient of the positive column, relatively low heat loss, low vapor pressure at ambient conditions and relatively low cost—precluded the search for other materials that would provide appropriate buffer gases for automotive HID lamps. Simply removing the mercury is inappropriate because the electrical and thermal conductivities of the arc must be controlled. The ideal replacement for mercury would have a large momentum cross-section, a high neutral particle density at temperature and high excitation and ionization energies.
The first two of these goals for a mercury replacement address the need to limit the discharge current at a given lamp power by increasing the resistance of the plasma sufficiently. Large excitation and ionization energies are required since the replacement should not dominate the visible spectrum significantly, that is, only transitions between high lying energy levels are possible. In addition to these physical properties, the chemical stability of the metal halide salts, electrodes and the quartz walls must be guaranteed for a few thousand hours. Finally, the replacement should be environmentally friendly.
Currently, the EU and the Japanese Electrical Lighting Manufacturers Association (JELMA) are considering amending Regulation 99 to include automotive mercury free HID lamps. The proposed EU and JELMA specifications for automotive mercury type “R-type” HID light sources, D1R, D2R, had the following proposed characteristics: rated voltage of the ballast 12 volts, rated wattage 35 watts; objective lamp voltage 85 volts, +/−17 volts; lamp wattage 35 watts +/−3 watts; luminous flux 2800 lumens +/−450 lumens; color coordinates (x=0.375, y=0.375) with a tolerance of (x≧0.345, y≦0.150+0.640x) and (x≧0.405, y≦0.050+0.750x). The corresponding mercury free D3R and D4R lamps were the same in each instance, except the objective lamp voltage was 42 volts +/−9 volts. The proposed EU and JELMA specifications for automotive mercury type “S-type” HID light sources, D1S, D2S, were the same in each instance as the D1R and D2R lamps, except the luminous flux was to be 3200 lumens. The corresponding mercury free lamps, D3S, D4S were the same in each instance as the D3R and D4R lamps, (lamp voltages 42 volts +/−9 volts) except the luminous flux also was to be 3200 lumens. As can be seen, the proposed performance requirements for the mercury free lamps, except for operating voltages, are identical to the mercury containing lamps. The requirement that the arc bending and diffusion be the same may significantly limit the choices of voltage increasing chemistries. The other differences between the D1/D2 (mercury containing) and D3/D4 (mercury free) lamps are an increase from <0.3 millimeter to <0.4 millimeter in electrode diameter (to allow for higher currents) and the keying of the bases to insure the light sources are not interchangeable.
Screening tools for potential mercury replacements are known. It has been asserted that the inclusion of a metal halide whose ionization potential (Vi) is between 5 and 10 eV and whose vapor pressure is at least 10−5 atmospheres at the lamp operating temperature will sufficiently raise the operating voltage of an automotive HID lamp without significantly increasing the rare gas pressure, K. Takahashi, M. Horiuchi, M. Takeda, T. Saito and H. Kiryu, U.S. Pat. No. 6,265,827 (2001). It is further asserted that electrode losses are reduced and the blackening of the arc tube due to electrode sputtering is suppressed. If the metal halide additive has an ionization potential <5 eV, the operating voltage of the lamp decreases; if the ionization potential is >10 eV, the lamp efficacy decreases; if the vapor pressure at the operating temperature is >10−5 atmospheres, an increase in the operating voltage is not observed.
One place to look for mercury replacements is in the same periodic family: cadmium and zinc. Cadmium is not a viable candidate since it is toxic and is being phased out of vehicle lighting, for example, amber turn signal lamps. The life of the lamps containing zinc will decrease because of the vigorous attack on the quartz at the higher operating temperatures required to obtain a sufficiently high vapor pressure (particle density). Work in higher wattage ceramic metal halide lamps suggests a reduction in efficacy of about 8%, a reduction in lamp operating voltage of 25% with a lower arc core temperature, and higher wall temperature when zinc is substituted for mercury, M. Born, Mercury-Free High Pressure Discharge Lamps, Paper 002:L, 9th International Symposium on the Science and Technology of Light Sources, Cornell University, Ithaca, N.Y., Aug. 12-16, 2001. In addition, the strong affinity of zinc for iodine effectively scavenges iodine from the metal halides, reducing them to elemental metals, M. Born and U. Niemann, Interaction of zinc with Rare Earth Halides Under Conditions of High Pressure Discharge Lamps, 10th International IUPAC Conf. on High Temp. Materials Chemistry, Apr. 10-14, 2000, Forschungzentrum, Julich, Germany. The lifetime of lamps at elevated temperature in the presence of aggressive metals (scandium or rare earths) is not expected to be sufficiently long for automotive applications.
Another place to look for a replacement is in the metal halides. Generally, the choices fall into two broad categories: additives that constrict the arc and additives that fatten the arc. The quality and stability of the arc in automotive HID lamps is more critical than in normal metal halide lamps. The automotive HID lamp is an optical source with strict requirements for arc position, arc bending and arc diffusion. Arc constricting chemistries have the advantage of tending to increase the lamp operating voltage. However, in constricted arcs convection carries the arc upward toward the top of the arc tube where severe localized heating can occur and very constricted arcs tend to be unstable. Thorium iodide (ThI4) and excess iodine (I2) have historically yielded constricted arcs. Many of the spectrally rich metals yield lamps with poorly wall-stabilized arcs. The poor quality of these arcs results from the metal having many energy levels, a number of which are quite low-lying, so that the average excitation potential is quite low relative to the ionization potential (Vavg<Vi/2). Alkali metal iodides are typical of arc fattening additives. Alkali metals have a low ionization potential and this has the effect of making electrons available in low-temperature regions of the arc. The presence of these electrons allows for electrical current flow, which in turn leads to power dissipation and more heat generation in these regions. The net effect is to raise the temperature locally and increase the diameters of the high-temperature region of the arc and of the electrically conducting region. As a result, the arc current for a given wattage increases and the operating voltage decreases. The addition of alkali to the quartz arc tube is possible only as iodides because the metals would react vigorously with the wall at the lamp operating temperatures.
The addition of gallium, indium and thallium iodides alone or in combination does not, in general, result in constricted arcs. The energy levels of these metals are more like those of mercury in that there are relatively few of them and most of them are of energy greater than or equal to half the ionization potential. This would predict wall-stabilized arcs, and also hold the promise of voltage enhancement.
It is possible to use these higher vapor pressure additives in combination with rare earth halides to produce chemical complexes within the lamp. The chemical complexing increases the number density of the radiating species, provides some buffering against wall reactions, and could also enhance the voltage drop across the column, W. P. Lapatovich and J. A. Baglio, Chemical Complexing and Effects on Metal Halide Lamp Performance, Paper 026:I, 9th International Symposium on the Science and Technology of Light Sources, Cornell University, Ithaca, N.Y., Aug. 12-16, 2001. The result would be a rare earth complex chemistry, for example, DyI3 with InI. However, the addition of complexing agents can have unintended consequences such as a shift in color coordinates as seen in FIG. 1. FIG. 1 shows the effect of metal iodides on the color coordinates (CCX, CCY) of a mercury free, rare earth chemistry. the ploygon repersents the boundary of the SAE white region.
Considerable effort has been expended in recent years to produce mercury free lamps that operate at high voltages so they can be used as retrofits with existing ballasts. Examples of art where high doses of metal additives are used to elevate the voltage are taught by Ishigami et al. in EP 0 883 160 A1, by Takeda et al. in EP 1 032 010 A1 and Uemura et al. in EP 1 150 337 A1. Examples of other voltage enhancing additives are taught by Takahashi et al. in EP 1 172 839 A2, and by Takahashi et al. in U.S. Pat. No. 6,265,827. Examples of high efficacy fills of a corrosive or toxic nature are taught by Kaneko et al. in EP 1 172 840 A2.
The use of zinc iodide in discharge lamps is known. See, for example, U.S. Pat. Nos. 4,766,348; 5,013,968; 4,992,700; 4,678,960; and 4,360,758. However, there is no suggestion in these references to use a particular amount of zinc iodide as a substitute for mercury in the lamp.